1. Field of the Invention
The present invention relates to an engine control apparatus, more particularly to an engine control apparatus for controlling the ignition timing in a lean-burn engine in which an air/fuel ratio is switched between the stoichiometric point and a lean point.
2. Description of the Prior Art
The conventional lean-burn engine is controlled such that the fuel is burned at a lean air/fuel ratio (ex. 20 to 25) far higher than the stoichiometric point (14.7) at a low engine speed or a light load condition, thereby improving the fuel consumption. While, the engine is controlled such that the fuel is burned at the stoichiometric air/fuel ratio under acceleration or heavy load. As a result, it is possible to improve the fuel consumption while maintaining the output torque.
If the predetermined air/fuel ratio is varied substantially when the lean air/fuel ratio and the stoichiometric air/fuel ratio is switched on the basis of the change in operation of the above-mentioned engine, a large torque difference is occured when the predetermined air/fuel ratio is switched, because the produced torques are different to each other even if the operating condition is the same. Therefore, in order to prevent the deterioration of drivability, the air/fuel ratio is not switched at a time, but it is varied in stages every air-intake stroke of cylinders and finally switched completely. The above lean-burn engine is disclosed in, for example, Japanese Patent Application Laid-Open No. 5-71381 (1993).
Now, in the above conventional control system of the air/fuel ratio for an engine, it is required to obtain the target air/fuel ratio through the medium air/fuel ratio (A/F=16 to 18) when switched between the stoichiometric point (A/F=14.7) and a lean point (A/F=16 to 18).
However, as described below, it is proved that the concentration of NOx in the exhaust gas at the medium air/fuel ratio between the stoichiometric point and the lean point is higher than that in exhaust gas at the lean air/fuel ratio and the stoichiometric air/fuel ratio. Therefore, if the fuel is burned at the medium air/fuel ratio, there is the problem that the concentration of NOx inreases. One disadvantage of the conventional engine control apparatus is that particularly in a large emission vehicle, it become difficult to clear the auto-emission standards of the total emission amount of NOx at a mode operation.
Even if the fuel injection amount is controlled in such a way that the target air/fuel ratio is varied step by step between the stoichiometric point and the lean point not so as to match with the medium air/fuel ratio, there is the cylinder in which the fuel is burned at the medium air/fuel ratio. Therefore, another disadvantage of the conventional engine control apparatus is that the concentration of NOx may become several times to several tens of times more than that in a steady state.